The object of the present invention is a process for dyeing textile materials constituted by polyamides, in particular quick dyeing polyamides.
Particular reference is made herein to polyamide 6 or nylon 6 (polycapronamide), but the invention is applicable to other types of polyamides, such e.g. polyamide 66 or nylon 66 (polyhexamethyleneadipamide) provided that they are quick dyeing.
What is meant by "quick dyeing polyamides" will be specified hereinafter.
As is known, a good dyeing of textile materials requires that the phenomena connected with the dyeing, such as the dye take-up by, and the migration of the dyes in the fibre of the material, occur in an optimal way. Said phenomena are influenced by a considerable number of variants, such as the nature of the material to be dyed, the composition of the dye bath, with respect to the dyestuffs and to other substances it may contain, temperature diagram of the dyeing process, the duration of its several stages, and so forth. Obviously it would also be possible, from a theoretical viewpoint, to obtain a good dyeing by controlling all the relevant factors at every moment of the process. But it is obviously impossible to do this on an industrial scale, because it would be necessary to program and control at every instant, with precision and delicacy, a series of factors, which would imply the adoption of different processes for each particular dyeing case and the detailed control of each process unit. It is therefore necessary to adopt processes which are as generally applicable as possible, viz. which are suitable, without variations, for dyeing the broadest possible range of textile materials. By particular reference to the polyamides, and specifically to polyamide carpet and rugs to which invention particularly, though not exclusively, refers, the art teaches that it it possible to adopt a general process which comprises a heating to high temperature, on the order of about 100.degree. C., because the fact that such temperatures are attained and maintained for a sufficient time at the end of the process, eliminates all the irregularities which may have intervened in the preceding stages. Thus, if in a certain stage different dyestuffs of the dyestuff mixture employed are taken up by the fiber or migrate therein with different speeds, the final high temperature stage equalizes their absorption and migration at least if suitably compatible dyestuffs are used. However, said high temperature dyeing process, which is normally adopted, has general drawbacks and particular drawbacks with reference to quick dyeing textiles. General drawbacks are essentially, on the one hand, substantial consumptions of heat and on the other, defects in the dyed fabric, especially when the piece winch dyeing process is used. In said process, which is often though not exclusively used and to which preferably, though not exclusively, the present invention refers, the carpet is kept in motion in the dye bath by means of a winch which rotates at a high, predetermined speed. In the dye bath the carpet is disposed in a series of folds which generate creases leading to the formation of corresponding marks (called, in the slang of the art, "stick marks"), due to the wet treatment at high temperature which produces a setting of the synthetic fibres, as is well known to persons skilled in the art.
Another drawback of the high temperature dyeing process is the length of its cycle, which comprises a long, slow heating stage up to 100.degree. C., a more or less long stage at 100.degree. C., and a slow, final cooling to 60.degree. C. In addition to these general drawbacks, it has been found that when quick dyeing polyamides are used, the dyed material may not become uniformly colored.
Low temperature processes are also known. However, none of them has proved satisfactory. It is found that the elimination of the high temperature leads to disuniformity in the absorption and migration of the dyestuffs and to an insufficient utilization thereof, or low dye yield, which it has been vainly attempted to eliminate by adding chemical dyeing aids to the dye bath. E.g., it has been proposed (CHEMIEFASERN/TEXTILINDUSTRIE Pag. E32, E33--MARCH 1979) to add to the dye bath ammonium thiocyanate together with decyl alcohol as wetting agent, but the results are not satisfactory, and further, the ammonium thiocyanate is somewhat dangerous because it is toxic and therefore is ecologically and operationally disadvantageous under the process condition, and the decyl alcohol leaves a persistent aromatic odor in the dyed materials. Other low temperature dyeing processes involve the use of different dyestuffs having different capacities of migration in the fiber in different stages of the process, but, as has been said, it is extremely undesirable from all viewpoints to adopt process which requires changing the composition of the bath in the course of the dyeing.